Adaptation of a high energy electron accelerator as a neutron source



Sept. 1, 1959 G. c. BALDWIN ETAL 2,902,613

ADAPTATION OF A HIGH ENERGY ELECTRON ACCELERATOR AS A NEUTRON SOURCE Filed April 9, 1954 7726/)" Attorney.

United States Patent ADAPTATION OF A HIGH ENERGY ELECTRON ACCELERATOR AS A NEUTRON SOURCE George C. Baldwin, Albany, and Erwin R. Gaerttner and Max L. Yeater, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Application April 9, 1954, Serial No. 422,009

2 Claims. (Cl. 31361) The present invention relates to charged particle accelerator apparatus and more particularly to an improved neutron source for use in an electron accelerator.

Apparatus for accelerating charged particles by means of magnetic induction effects is shown and described in the United States Patent Nos. 2,394,071, 2,394,072 and 2,394,073, all of which were patented February 5, 1946 by Willem F. Westendorp and assigned to the assignee of the present invention. Such apparatus can comprise a core of magnetic material including a pair of opposed, rotationally symmetrical pole pieces which define an annular gap in which a toroidal evacuated container is positioned. The core is excited by means of windings that are energized by a source of time-varying voltage to produce a time-varying magnetic flux which links an equilibrium orbit within the evacuated container and a time-varying magnetic guide field which traverses the equilibrium orbit. Charged particles, e.g. electrons injected along the equilibrium orbit from an electron gun position adjacent to the orbit Within the region of influence of the time-varying magnetic guide field, are accelerated to high energy levels by the time-varying magnetic flux during a great number of revolutions while the time-varying magnetic guide field constrains the particles to follow paths along the equilibrium orbit. After acceleration to a desired energy, the electrons can be diverted from the equilibrium orbit to a target for the generation of X-radiation and the subsequent production of neutrons.

One method of obtaining neutrons is that of placing an appropriate target in the X-ray beam emerging from an accelerator. In the practice of this method there is generally more than one actual neutron source since neutrons are produced at the X-ray target and at the exit port of the vacuum tube portion of the accelerator as well. Also, the X-ray beam is permitted to emerge from the accelerator thereby giving rise to background X-radiation. Furthermore, the physical dimensions of such a target must be large so that a major fraction of the emergent X-ray beam can be intercepted. It is therefore an object of this invention to produce neutrons by combining the X-ray and neutron sources in the same physical object, located'within the vacuum tube envelope of the accelerator, and of such geometry that a maximum flux of neutrons is produced with a minimum external flux of X-radiation. By this means, a single source of high intensity but small physical extent can be realized.

Another object of our invention is to provide a method and apparatus for producing a short time duration pulse of neutrons.

A further object of our invention is to provide an im proved source of fast and slow neutrons.

According to one aspect of our invention, a target is inserted into the evacuated portion of a high energy electron accelerator. This target is partially surrounded by a piece of moderator material. The beam of high energy electrons is caused to strike the target by means of well-known orbit shifting methods. The electron beam causes X-rays to be produced in the target which,

The features of the invention desired to be protected herein are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Figure 1 is a simplified, partially sectionalized view of a magnetic induction accelerator apparatus which may be used in the practice of our invention; Figure 2 is a partially sectionalized View of the vacuum portion of the accelerator of Figure 1 showing the incorporation of the neutron source in our invention; Figure 3 is a detailed view of the neutron source and mounting structure; and Figure 4 is a sectional view of the target along section line 44 of Figure 3.

By Way of example, Figure 1 shows a magnetic induction accelerator apparatus suitably embodying the invention. The apparatus comprises a magnetic core 1 which can be laminated to minimize the generation of eddy currents therein. Core 1 includes laminated, rotationally symmetrical, opposed pole pieces 2 and 3 having generally outwardly tapered pole faces 4 and 5 for the provision of a magnetic guide field traversing an equilibrium orbit O, as will be more fully described hereinafter. Coaxial with pole pieces 2, 3 and disposed between pole pieces 4, 5 is an evacuated annular container or envelope 6 of dielectric material, which provides within its interior an annular chamber 7 wherein charged particles can be accelerated. The central portions of pole pieces 2, 3 are terminated respectively by fiat surfaces 8, 9 between which are disposed laminated ferromagnetic disks (not shown) and dielectric support spacers 10, 11. The ferromagnetic disks serve the purpose of reducing the reluctance of the magnetic path in a region between surfaces 8 and 9.

Magnetic core 1 can be excited from a suitable source of time-varying voltage 12 connected as indicated to series-connected energizing windings 14, 15 surrounding pole pieces 2, 3. To minimize the current drawn from source 12, energizing windings 14 and 15 can be resonated by power-factor-correcting capacitors 16. Within chamber 7 adjacent to equilibrium orbit O and also within the region of influence of the time-varying magnetic guide field existing between pole faces 4, 5 during operation of the apparatus, there is provided a charged particle source 17 which is supported from a hermetically-sealed side arm 18 of envelope 6. A more detailed illustration and description of electron gun structures suitable for the present purposes can be found by reference to the above-mentioned patents, or by reference to the United States Patent No. 2,484,549 of J. P. Blewett, patented October 11, 1949 and assigned to the assignee of the present invention.

It is well understood by those familiar with magnetic induction accelerator apparatus that energization of windings 14, 15 by the source of the time-varying voltage 12 results in a time-varying magnetic flux which traverses magnetic core 1 and pole pieces 2, 3 to provide a timevarying magnetic flux that links equilibrium orbit O and a time-varying magnetic guide field that traverses the locus of equilibrium orbit O and the vicinity thereof between pole faces 4, 5. Electrons emitted by gun 17 at a desired timed instant near zero value in the cycle of magnetic flux and field variations are continuously accelerated during the acceleration portion of the cycle so that they execute repeated revolutions along equilibrium orbit O. The injected electrons can be caused to assume energies of many millions of electron volts and then can be automatically diverted from the equilibrium orbit by means of pulsatingly energized orbit shift coils 19, and 2t). Circuits for the proper time-injection and subsequent diversion of the charged particles from the equilibrium orbit at or near the end of the acceleration cycle are disclosed in the aforementioned patents and additionaily in the United States Patent No. 2,394,070 of. D. W. Kerst, patented February 5, 1946 and assigned to the assignee of the present invention.

After the charged particles have been accelerated to a desired energy level, they must be diverted from the equilibrium orbit to permit useful utilization of the energy which has been imparted to them. Diversion of the charged particles from the equilibrium orbit can be accomplished by supplying a properly timed pulse of current to orbit shift coils 19 and 20. The application of the current pulse to the orbit shift coils modifies the magnetic induction throughout the stable regions surrounding equilibrium orbit O and causes the charged particles to spiral inwardly or outwardly from the equilibrium orbit, depending upon the direction of the flux generated by the orbit shift coils with respect to the flux generated by the windings 14, 15. During their travel away from the equilibrium orbit the charged particles can be considered as following paths tangent to their respective instantaneous circles the radii of which are gradually increasing or decreasing as the case may be.

According to the present invention a single target may be utilized to produce neutrons. Figures 2, 3 and 4 show in varying degrees of detail the structure and mounting apparatus for applicants novel neutron source. In these figures like members are designated by the same reference numbers. Figure 2. illustrates a portion of the evacuated glass tube structure of the accelerator of Figure 1. The circular glass tube portion of the accelerator has a neck-like portion 21 to which is attached a flange 22. Attached to flange 22 is sylphon structure 23 which is illustrated in greater detail in Figure 3. The sylphon structure consists of copper bellows 24 and adjusting bolts 25-28 which are headed in flange 29. Flange 29 is hermetically sealed and mechanically attached to flange 22. The other end of copper bellows 24 is hermetically sealed to aluminum flange 30. Rigidly attached to flange 30 is goose-neck arm 31 to which target structure 32 is rigidly attached. Flange 30 has the center portion thereof machined down to form a very thin wall through which neutrons can pass with minimum attenuation. The thickness of the Wall is determined by the mechanical strength required to withstand the atmospheric pressure. It is noted that the pressure within glass tube 6 is generally in the neighborhood of to 10* microns. The radial position as well as the circumferential position of the target may be readily adjusted by taking up or slacking off the nuts attached to bolts 25-28. In this fashion, the position of the neutron source 32 may be easily adjusted to permit free acceleration of particles in the accelerator and proper positioning so that the accelerated stream of charged particles may be diverted to strike the target.

A detailed illustration of the neutron source is shown in Figure 4 of the drawing. Neutron source 32 consists of target 33 which is made up of alternate layers of suitable target material and thin layers of an insulating material such as mica. The target material is laminated in order to reduce the effect of eddy currents which would tend to interfere with the operation of the charged particle accelerator. The stack of target metal is retained in a hemisphere 34 of hydrocarbon plastic material which acts as a moderator for neutrons generated in the target 33. A bolt 37 of target material is formed with X-radiation.

a flat head 35 and secures the target in the hemisphere of hydrocarbon plastic material by means of a nut 36 attached to the end of bolt 37. Goose-neck 31 is secured in the hydrocarbon plastic hemisphere and is mechanically connected to target 33 by means of a metal plate 31' to provide a heat conducting path from the target to the exterior of the accelerator. The hemisphere is provided with an accessed way 38 through which the stream of electrons may be directed in order to produce neutrons. The entire external surface of the hydrocarbon plastic body is covered with a conductive coating 39 electrically connected to goose-neck 31 to prevent the build up of charge on the target and plastic body. The gooseneck 31 is made of thin hollow non-magnetic tubing. Since the goose-neck 31 is hollow, it tends to reduce eddy currents and also provides a means of removing heat which is developed in target 33 by absorption of the energy of the accelerated electron beam.

In order to further describe the selection of satisfactory target maerial from which neutrons may be produced by bombarding it with high energy electrons it is necessary to define two terms which are well-known to those skilled in the art. The terms to be defined are (1) a radiation unit and (2) threshold energy for neutron production.

A radiation unit is a convenient means of defining the thickness of material when discussing radiation phenomena. A radiation unit may be defined as that thickness of a given material for which the probability that one electron will undergo a radiation producing collision as it passes through the material is 1/ e, where e is equal to 2.718. For example, a piece of material which is 0.25 radiation units thick will, when a high energy stream of electrons is directed thereon, pass 1e or approximately of the electrons through the material without a radiation producing collision occurring. Approximately one quarter or r, of the high energy particles or electrons impinging thereon will result in the production of A radiation unit may be mathematically defined as that thickness X for which =4Z 31 3 1n 183Z The term Z is the effective atomic number of the target including the effect of the extra-nuclear electrons, defined as (Z -l-Z) r is the classical electron radius,

N is Avogadros number, 6.023 10 mol- A is the atomic weight of the target; and Z is the atomic number of the target substance.

It has been found that, although as the thickness of the material is increased the probability of radiation producing collisions will increase, a diminishing fraction of this radiation will emerge, since the material itself tends to absorb the X-rays generated therein. It is well established that a balance is reached when a piece of material is approximately 0.33 radiation units thick. With material of this thickness, a balance is reached between the production and the absorption of X-rays.

The threshold energy for neutron production is that minimum energy of X-ray quanta required to produce a nuclear reaction in which a neutron is emitted. Depending upon the particular material bombarded, high energy X-rays must have been generated by electrons having more than this certain minimum energy in order to photodisintegrate a target nucleus and cause the ejection of a neutron from the target nucleus. The threshold energy levels have been worked out for many elements. The threshold energy for generating neutrons by meansv of X-rays is near 8 Mev. in most cases. An exception is beryllium for which the threshold is in the neighborhood of 1.6 Mev. In order to obtain neutrons the electrons must strike the target with at least the threshold energy. The production of neutrons will increase rapidly as the electron energy is increased above the threshold.

The selection of satisfactory target material for the target 33 of applicants neutron source is dependent largely upon the above two described phenomena. A very satisfactory target may consist of alternate layers of uranium and thin sheets of an insulating substance such as mica. Since a radiation unit of uranium is approximately /3 of a centimeter, uranium is very satisfactory as a target material in an accelerator generating electrons of energies well in excess of 6 Mev.

Beryllium has a threshold energy level in the neighborhood of 1.6 Mev. and, therefore, would appear to make a satisfactory target material. However, a radiation unit of beryllium is approximately 25 centimeters and therefore the target would tend to become prohibitively large in many of the conventional particle accelerators. In order to take advantage of the small radiation unit of uranium and the low threshold energy level of beryllium a satisfactory target may be made up of alternate layers of uranium and beryllium. A-satisfactory target may be formed of any element or elements provided the considerations of threshold energy level and radiation thickness are complied with to provide a' target several radiation units thick and having a sulficiently low threshold energy level for the particular accelerator in which it is to be employed. V

In the particular embodiment of applicants invention the target was made up of successive layers of uranium separated by thin layers of mica. The target is approximately inch by inch by inch. The uranitun layers are approximately 20 mils thick separated by mica shims, approximately 1 to 2 mils thick. The target is held in the hydrocarbon plastic body by means of uranium bolt 37, as has been previously described. It will be noted that the target is several radiation units thick thereby providing a satisfactory source of neutrons with a minimum of external X-radiation. The moderator structure surrounding the target is illustrated as being in the form of a hemisphere. The purpose of the moderator is to provide a means of reducing the energies of a portion of the neutrons generated in target 33, in order to provide low energy neutrons. The moderator structure may be made out of any one of the well-known moderating substances (viz. hydrogenous) such as polymethyl methacrylate, polystyrene or similar hydrocarbon materials, and should be so shaped that it does not intercept the equilibrium orbit or interfere with the operation of the particle accelerator.

When orbit shift coils 19, 20 are energized with a pulse of current to cause the charged particles to spiral inwardly after they have been accelerated to a desired energy level as described in the aforementioned Patent No. 2,394,070, the beam of charged particles strikes the edge of target 33 by passing through slot 38 in the moderator hemisphere. The high energy electrons when they strike the target produce X-rays. The X-rays interact with atoms of uranium to produce a shower of electrons at an energy level below the initial energy level of the electron stream. These electrons produce more X-rays and the X-rays more electrons of a still lower energy level until the energy of the incident electron beam has been completely dissipated. As explained above, some neutrons are produced by the X-rays. These neutrons are emitted in a random, isotropic fashion. All of the neutrons produced in the above-described fashion may be considered fast neutrons. The moderator structure is designed to provide some slow neutrons. It should be noted that the moderator structure is necessary only if some slow neutrons are desired. In the illustrated neutron source certain of the neutrons will travel in directions which will intercept hydrogen or carbon nuclei. These neutrons will bounce off the hydrogen or carbon nuclei and lose energy. The neutrons may be involved in a number of collisions so that the neutron energy is either completely absorbed or until the neutron is emitted in a direction so as to emerge from the moderator. A

portion of these neutrons will pass through the thin center wall section of flange 30. In this manner, a certain number of slow neutrons will be produced along with the fast neutrons which travel directly from the target through flange 30. Those neutrons which leave the target or moderator structure in a direction substantially in the plane of the equilibrium oribit and perpendicular to the path of the equilibrium orbit will pass through the thin center wall section of flange 30 and may be utilized. The other neutrons and other radiation products will be absorbed by the glass structure or other neutron absorbing materials such as external water walls.

In this particular adaptation of applicants neutron source, the source of high energy particles is a betatron providing an electron stream at an energy level of approximately Mev. The electrons strike the target at substantially the 100 Mev. level and generate showers therein. The energies of the particles in-the shower are gradually degraded by successive collisions. In the process of this degradation neutrons are produced which have kinetic energies in a wide range averaging approximately 1 Mev. If appreciable slower neutrons are required, the moderator structure 34 must be incorporated with the target 33.

A neutron source in accordance with our invention has been found to provide a satisfactory short pulse of neutrons for use in time of flight measurements. The time for a neutron to travel a given distance is a direct indication of the energy of such a neutron. By utilizing applicants novel target it has been possible to produce a pulse of neutrons with a duration in the neighborhood of /3 of a microsecond. This permits the time of flight to be measured to high accuracy. The production of the pulse of neutrons may be further visualized in the following manner. The electron beam is deviated from the equilibrium orbit by means of coils 19 and 20 through which a high current is passed. This high current pulse causes the beam to cross the target material. As soon as the full cross-section of the beam, of the order of millimeter, has intersected the target the beam is completely extinguished and the neutrons are produced almost instantaneously.

It is noted that the utlization of a relatively thick target decreases the external X-ray beam intensity. As the optimum thickness of approximately .33 radiation units is exceeded the production of internal X-rays is increased and therefore the number of neutrons produced is increased. However, the additional X-radiation will be absorbed in the target itself while the additional neutrons will emerge and may be utilized. The neutron yield which can be achieved with this type of target is much greater than can be obtained with separate X-ray and neutron targets, since the thickness of the former should not exceed 0.33 radiation units, while the thickness of the combination target is not so limited.

In view of the foregoing, it may be seen that we have provided a novel form of neutron source which provides the greatest possible yield of neutrons, a minimum of external X-ray flux, a relatively small physical size of neutron source and no extraneous sources of neutrons. Therefore, this source of neutrons, in accordance with our invention, provides a large number of neutrons having a wide range of energies with a minimum of stray radiation.

From the foregoing description it will be readily appreciated that the present form of our invention is given merely by way of example and that a neutron source in accordance with our invention may take a variety of forms. Moreover, the present invention is not limited to utilization in conjunction with accelerator apparatus which employs magnetic induction phenomena alone, but can also be used with synchrotron apparatus such as that disclosed in United States Patent No. 2,485,409, patented October 8, 1949, by Willem F. Westendorp and Herbert C. Pollock and assigned to the assignee of the present invention. The present invention has application in any ap- 7 paratus providing a high energy source of electrons which can be caused to impinge upon the target material;

What We claim as new and desire to secure by Letters Patent of the United States is:

1. In an accelerator having an evacuable envelope and means providing a stream of high-energy electrons of predetermined mean energy within said envelope, a source of neutrons comprising target means located Within said envelope and including interspersed first and second materials each producing X-rays upon bombardment by said electron beam, said first material having a threshold energy value for the production of neutrons by X-rays less than said predetermined mean energy, and said second material having a threshold energy value for the production of neutrons by X-rays greater than said predetermined mean energy, the X-ray radiation unit for said second mateiral being less than the X-ray radiation unit for said first material, said target means as measured along the direction of said beam being substantially greater than one-third X-ray radiation unit in thickness, and means causing said beam to strike said target to produce neutrons by the X-ray-neutron reaction.

2. A target for producing neutrons when subjected to bombardment by a high-energy electron beam of predetermined mean energy, said target comprising inter- 2 spersed first and second materials, each producing X-rays upon bombardment by said electron beam, said first material having a threshold'energy value for the production of neutrons by X.rays less than said predetermined mean energy, and said second material having a threshold energy value for the production of neutrons by X-rays greater than said predetermined mean energy, the X-ray radiationunit for'said second material being less than the X-ray radiation unit for said first material, said target as measured along the direction of said beam being substantially greater than one-third X-ray radiation unit in thickness to enhance the production of neutrons by the X-ray-neutron reaction.

References Cited in the file of this patent UNITED STATES PATENTS 2,161,985 Szilard June 13, 1939 2,287,619 Kollmann et al June 23, 1942 2,287,620 Kollmann et al. June 23, 1942 2,335,014 Kerst Nov. 23, 1943 2,549,596 Hamilton et al Apr. 17, 1951 2,640,924 McMillan June 2, 1953 OTHER REFERENCES Feld: Nucleonics, vol. 9, No. 4, October 1951, pp. 51-57.

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